Conversion of hydrocarbon oils



Nov. 2, 1937. J, w, man-REY f 2,098,057

CONVERSION OF HYDROCARBON OILS Original Filed May 23, 1934 FRACTIONA'GfiE COLUMN Patented Nov. 2, 1937 7' I UNITED STATES PATENT OFFICE CONVERSION OF HYDROCARBON OILS John Warren McCaifrey, Chicago, Ill., assignor to Universal Oil Products Company, Chicago, 111., a corporation of Delaware Application May 23, 1934, Serial No. 727,070

Renewed September 20, 1935 g 1 Claims. (Cl. 196-48) This invention particularly refers to an imsubjected to a high rate of heating, so that they proved process for the fractional distillation of may be quickly brought to the desired conversion hydrocarbon oils of relatively wide boiling range temperature without being maintained at or near accompanied by the selective conversion of relathe maximum conversion temperature attained 5 tivelylow-boiling and relatively high-boiling fracfor a sufficient length of time to cause their extions of the charging stock, together with correcessive conversion, and the relatively low-boiling sponding low-boiling and high-boiling fractions oils to be subjected to conversion are caused to of the intermediate conversion products of the pa s fi st through e flu d uits located dprocess. jacent the roof of the combustion zones, wherein In the present invention the heat required for they are heated to a relatively high conversion 10 the topping and cracking operations is derived temperature at a somewhat slower rate of heating from a single furnace structure and as a special than that employed for the relatively high-boilfeature of the invention the arrangement of the ing oils (sin less r pid heating is not objectionheating coils or fluid conduits within the furnace able for the low-boiling oils), the highly heated through which the various oils are passed is such low-boiling oilsbeing then maintained at or near that each separate stream of oil is subjected to the maximum conversion temperature attained the heating conditions most suitable for its treatfor a p det rm ed time y passing them ou ment. that portion of the fluid conduit within the cen- In its preferred form the furnace whichIemploy trally located fluid heating zone which is exposed is of the general type having two separately fired to the relatively hot combustion gases from the 20 combustion and heating zones with fluid conduits ombustion zones of the furnace. located along the roof and along the floor of each These conditions of treatment, na y. rapid and with a centrally located fluid heating zone a d e at e y s o t e heating or h-b ili supplied with combustion gases from both com- Oils a d more Prolonged heating to SOmeWhat bustion zones and containing another fluid conhigher ma mum temp rature a mpani d by 5 duit. It has been found that, in furnaces of this Seeking for the low-boiling 0118 a been found general character, the tube banks adjacent the most suitable for the production of high yields of floor of the combustion zones are subjected to the desirable light liquid products, such as motor fuel most severe heating conditions while somewhat of high anti-knock value, W h a minimum o es milder heating conditions ordinarily prevail in the e able P d c s u s e a d 30 fluid conduits adjacent the roof of the combus- The topp 0011 es a a nom z to tion zones and the mildest heating conditions are tract the excess residual heat from the combusobtained about the fluid conduit located in the tion gases having passed over the soaking section central or fluid heating zone of the furnace, of the fluid conduit in the centrally located fluid In the present invention advantage is taken of heating zone of the furnace. This residual heat 35 the normal heating tendencies of this type of furis normally ample to heat the charging stock sufnace'by arranging the flow of the various oils to ficiently to accomplish its fractional distillation, be treated through the different fluid conduits although, when r qu r d. addit al h atin of within the furnace in such a manner that each the charging stock may be accomplished iniany 40 type of oil to be subjected to different conditions well known manner. of treatment is also subjected to the heating'con- The term soakinas here used is intended to ditions most suitable for that type of treatment to designate e lat e es of heating o the T619;- which the particular oil is best suited. This is actively l w-boilin s w r h y a maincomplished by passing the charging stock for the tained at or near the maximum conversion temprocess, which is to be heated to a temperature perature previously attained for a predetermined 45 suflicient to effect its fractional distillation under time.

' conditions which preclude any appreciable con- It will be apparent from the foregoing that version, through that portion of the fluid conduit there is a definite cooperative relation in the within the centrally located fluid heating zone present invention between the selectivity of the which is heated by contact with partially cooled cracking operation, the type of oil undergoing combustion gases prior totheir passage from the treatment and the heating conditions employed furnace, while the relatively high-boiling oils to for distillation of the charging stock and for the be subjected to conversion are caused to pass conversion of each selected fraction of the chargthrough the fluid conduits locatedv adjacent the ing stock and reflux condensate. I am aware that 1 floor of the combustion zones wherelnthey are topping and cracking processes and selective cracking processes have been previously employed. I am also aware that furnaces of the general type described are now in use. The present invention, however, provides a novel and advantageous combination of various features and steps which, although well known in themselves, mutually contribute to produce advantageous results which have not heretofore been obtainable.

The accompanying diagrammatic drawing illustrates an apparatus in which the process may be carried out.

Referring to the drawing, furnace I comprises two similar combustion zones 2 and 2' each of which are independently fired by means of firing turmels such as 3. Combustible fuel, together with a regulated portion or all of the air required for combustion, is supplied to combustion zones 2 and 2 through ducts 4 in the firing tunnels by means of burners 5 of any suitable form, additional air being supplied, as desired, to the combustion zones through ducts 6 and I located respectively above and below the firing ducts 4 in each of the tunnels 3. A fluid heating zone 8 is located within the central portion of the furnace and is separated from combustion tunnels 2 and 2' by the respective-bridge walls 9 and 9'.

The combustion gases from both combustion zones pass over the respective bridge walls and downward through fluid heating zone 8 to flue ID from which they may pass to a suitable stack, not shown.

A fluid conduit or tube bank H comprising, in the case illustrated, a single horizontal row of horizontally disposed tubes I2, connected at their ends in series, is located adjacent the floor of combustion zone 2 and a similar bank H is located adjacent the floor of combustion zone 2'. Tube banks II and II are subjected to heating predominantly by radiation from the materials undergoing combustion in the respective com'- bustion zones 2 and 2' and from the hot refractory walls of the furnace and are exposed to heating conditions normally more severe than those employed in any other tube banks of the furnace.

Afluid conduit or tube bank l3 comprising in the case illustrated, a single horizontal row of horizontally disposed tubes l2 connected at their ends in series, is located adjacent the roof of combustion zone 2 and a similar tube bank I3 is located adjacent the roof of combustion zone 2. Tube banks I 3 and I3 are subjected to heating by radiation from the materials undergoing combustion in the respective combustion zones 2 and 2' and from the hot refractory walls of the furnace and are also subjected to appreciable heating, particularly in the portions above bridge walls 9 and 3', by contact with the hot combustion gases. The average heating conditions in tube banks l3 and I3 are normally somewhat less severe than thoseto which tube banks II and l l are exposed but are still suificiently severe that a relatively high average rate of heat input is obtained in the tubes of these banks.

Two separate fluid conduits or tube banks I4 and I5, each comprising a plurality of superimposed horizontal rows of horizontally disposed tubes, connected at their ends in series, are located within fluid heating zone 8 of the furnace. The tube bank I 4 is located above tube bank l5 and is therefore exposed to more severe heating conditions since the combustion gases from combustion zones 2 and 2' must pass over the tubes of bank I 4 before they reach bank I 5. The average heating conditions in each of these banks is,

however, relatively mild as compared to the conditions prevailing about the floor and roof banks although a portion of tube bank l4, particularly the upper two rows of tubes, is subjected to heating by radiation as well as by contact with the hot combustion gases. 1

Hydrocarbon oil charging stock for the process which may comprise any desired hydrocarbon oil of relatively wide boiling range, specifically including crude petroleum, may be supplied through line 3| and valve 32 to pump 33 by means of which it is fed, in the case here illustrated, through line 34 and valve 35 to tube bank l5, wherein it is subjected to relatively mild heating conditions and brought to a temperature sufficient to effect its substantial vaporization and fractional distillation without being subjected to any appreciable pyrolytic conversion. In the case here illustrated, the charging stock enters the lowermost row of tubes of bank l5, passing in series through adjacent tubes in each row and through adjacent rows of tubes in a. general direction countercurrent to the direction of flow of the combustion gases, being discharged from the uppermost row of tubes of bank I 5. It is, however, within the scope of the invention, when desired, to pass the oil through the tubes of bank IS in any other well known manner. The charging stock may, of course, be preheated, when desired, prior to its introduction into heating coil l5, by means of heat recovered from within the system, or in any other well known manner, not

illustrated.

into selected relatively low-boiling and high-boiling fractions.

' When the charging stock contains any desirable low-boiling fractions such as, for example, motor fuel of satisfactory anti-knock value, this material may be removed as fractionated vapors from the upper portion of column 38 through line 39 and valve 40 to be subjected to condensation and cooling in condenser 4|. The resulting distillate, together with any gas produced by the distilling operation may pass through line 42 and valve 43 to collection and separation in receiver 44. Uncondensable gas may be released from the receiver through line 45 and valve 46. The distillate may be withdrawn from receiver 44 through line 41 and valve 48 to storage or to any desired further treatment.

When the charging stock contains no appreciable quantity of low-boiling fractions which it is desirable to recover without conversion, its total low-boiling components to be subjected to conversion including, for example, any or all of the fractions comprising gasoline of poor antiknock value, naphtha, kerosene, kerosene distillate, light gas oiland the like, may be withdrawn as fractionated vapors from the upper portion of column 38, subjected to condensation in condenser I and collected in receiver 44, in the manner previously indicated, to be withdrawn therefrom through line 43 and valve 50 to pump 5| and directed therefrom through line 52, valve 53, line 51 and line 3| to conversion in heating coils l3, l3 and' I4, as will be later more fully described. In case low-boiling components of the charging stock, not to be subjected to conversion, are collected in receiver 44, its remaining relatively low-boiling components including, for example, any or all of the materials such as highboiling motor fuel fractions, naphtha, kerosene,

kerosene distillate, light gas oil, etc., may be separately withdrawn as one or a plurality of side streams from any .suitable intermediate point or plurality of points in column 38, for example, through line 54 and valve 55 to pump 56 wherefrom they may be directed through line 51, valve 58 and line 9| to conversion in tube banks l3, l3 and M, as will be later more fully described.

The remaining components of the charging stock, comprising its relatively high-boiling fractions which remain unvaporized or are condensed in column 38 and collect within the lower portion of this zone, may be withdrawn therefrom through line 59 and valve 60 to pump 8| to be supplied therefrom through line 52, valve 63 and line 85 to conversion in tube banks II and II, as will be later more fully described.

It is, of course, within the scope of the invention to separately recover any desired selected fraction or fractions of the charging stock from column 38, by well known means not here illustrated, particularly in case the charging stock contains valuable intermediate or high-boiling componentssuch as lubricating stock, asphalt, etc. The invention also contemplates preheating of the oils supplied to any heating coil of the furnace, when desired, by well known means not illustrated such as, for example, indirect contact and heat exchange with hot vaporous and/ or liquid products of the process.

In the case here illustrated the stream of relatively heavy oil supplied through line 85'to the tubes adjacent the fioor of the furnace is split into two streams of substantially equal volume which pass through lines 64 and 64' controlled by the respective valves 65 and 85' into tube banks II and II, respectively, fiowing in series through the adjacent tubes of each bank in a general direction concurrent to the direction of firing and being discharged therefrom through the respective lines 65 and 56', the two streams commingling in line 61 and passing through valve 68 in this line into reaction chamber 59. It will be understood, of course, that any other desired flow arrangement may be employed for the relatively heavy oil in heating coils II and into line 13, wherein they commingle, and enter the lowermost row of tubes of bank l i, flowing in series, through adjacent tubes in each row and through adjacent rows of tubes in a general direction countercurrent to the direction of flow of the combustion gases through zone 8, theheated stream of low-boiling oil finally being discharged from the uppermost row of bank l4 through line 14 and valve 15 into reaction chamber 69. The relatively low-boiling oils may, of course, when desired, be passed through tube banks l3, l3 and I4 in any other desired manner, however, tube bank I4 is preferably, in all cases, utilized as the final heating zone for the low-boiling oils.

A substantial superatmospheric pressure is preferably maintained at the outlet from both the light oil and the heavy oil heating coils and chamber 69 is also preferably maintained at substantial superatmospheric pressure so that conversion of the heated products supplied to this zone, and particularly their vaporous components, may continue therein. Although not indicated in the drawing, chamber 68 is also preferably well insulated in order to conserve heat. In the case here illustrated, both vaporous and liquid conversion products are discharged in commingled state from the lower portion of chamber 69 through line 15 and valve 11 into vaporizing chamber 18. It will be understood that, although not indicated in the drawing, the two streams of hot conversion products instead of entering chamber 59 in commingled state, as indicated in the drawing, may be separately introduced into this zone, each at any desired point and in any well known manner. It is also within the scope of the invention to separately withdraw vaporous and liquid conversion products from chamber 89, although the various well known means whereby this may be accomplished are not illustrated in the drawing. Also, when desired, chamber 18 may be eliminated, in which case residual liquid may be discharged from chamber 69 to cooling and storage or elsewhere, as desired, while the vaporous products separately withdrawn therefrom are subjected to fractionation, or the operating conditions may be so controlled that coke is produced in chamber 68 as the final residual product ofthe process.

A substantially reduced pressure relative to that employed in chamber 59 is utilized in chamber 18 in order to accomplish further vaporization of the liquid conversion products supplied to this zone. Residual liquid remaining unvaporized in chamber 18 may be withdrawn therefrom through line I83 and valve 156 to cooling and storage or elsewhere, as desired. It is also within the scope of the invention, when desired, to operate chamber 18 for the production of substantially dry coke in this zone, as the final residual product of the process.

In case coking is employed in either chamber 88 or chamber it a plurality of such zones may be employed, when desired, although not illustrated, to provide additional space for the deposition of coke and they may be simultaneously operated or, preferably, alternately operated, cleaned and prepared for further operation so that the duration of the operating cycle of the process is not limited by the capacity of the coking zone.

Vaporous products pass, in the case illustrated, from chamber 18 through line 88 and valve 8! to fractionation in fractionator 82, wherein their components boiling above the range of the desired light distillate conversion product of the process are condensed as reflux condensate. The reflux condensate is separated, in the case here illustrated, into selected relatively low-boiling and high-boiling fractions. The high-boiling fractions are withdrawn from the lower portion of fractionator 82 through line 83 and valve 84 to pump 85 wherefrom they are supplied through line 85 and valve 81 to conversion in the heavy oil heating coil, comprising tube banks II and H, together with the high-boiling oil from column 38, supplied to the heating coil as previously described. The low-boiling reflux condensate may be withdrawn from any suitable intermediate 7 point or plurality of points in fractionator 82, for example through line 88 and valve 89 to pump 90 by means of which this material is supplied through line 9! and valve 92 to further conversion in the light oil heating coil, comprising tube banks l3, l3 and I4, together with selected lowboiling fractions of the charging stock, supplied to this zone as previously described.

Fractionated vapors of the desired end-boiling point are withdrawn, together with uncondensable gas produced by the cracking operation, from the upper portion of fractionator 82 through line 93 and valve 94 and are subjected to condensation and cooling in condenser 95. The resultingdistillate and gas passes through line 96 and valve 91 to collection and separation in receiver 90. Uncondensable gas may be released from the receiver through line 99 and valve I00. The distillate may be withdrawn from receiver 98 through line I01 and valve I02 to storage or elsev where, as desired.

The usual expedients 'for assisting fractionation of the vapors in column 38 and fractionator 82 may, of course, be employed within the scope of the invention, although not here illustrated. Such expedients may comprise, for example, returning a regulated portion of the distillate from receivers 44 and 98 to column 38 and fractionator 82, respectively.

In an apparatus such as illustrated and above described, the preferred range of operating conditions may be approximately as follows: The conversion temperature employed at the outlet from the relatively heavy oil heating coil may range, for example, from 800 to 950 F., or thereabouts, preferably with a substantial superatmospheric pressure, measured at the outlet from the heating coil, of from 100 to 500 pounds, or thereabouts, per square inch. The heating coil to which the relatively low-boiling oil is supplied may utilize an outlet conversion temperature ranging, for example, from 925 to 1050 F., or thereabouts, preferably with a superatmospheric pressure at this point in the system of from 200 to 1,000 pounds, or-thereabouts, per square inch. The temperature employed at the outlet from the heavy oil heating coil is preferably within approximately 10, or thereabouts, of the maximum attained in this zone while the maximum temperature attained in the light oil heating coil is preferably within 30", or thereabouts, of the outlet conversion temperature from this zone. The temperature to which the charging stock is heated will vary considerably, depending upon its characteristics and the desired degree of separation to which it is subjected as well as upon the pressure conditions employed in the distilling stage of the process. The temperature of the charging stock entering the distilling and fractionating column may range, for example, from 400 to 750 F., or thereabouts. Any desired pressure from substantially atmospheric to 200 ,pounds, or more, per square men may be emaoaaos'r pounds, or thereabouts, per square inch down to substantially atmospheric pressure. The pressures employed in the fractionating, condensing and collecting portions of the cracking system may be substantially the same or somewhat lower than the pressure employed in the preceding stage of the process. Under normal operating conditions, with a furnace and a flow therethrough such as illustrated and above described, the relatively heavy oils may be subjected to an average rate of heating of the order of 12,500 to 17,500 B. t. u.s per square foot, per hour, while the average rate of heating to which the relatively low-boiling oils are subjected in the roof banks may range, for example, from 10,000 to 15,000 B. t. u.s per square foot, per hour. The average rate of heating in the soaking section of the light oil heating coil (the tube bank located within the upper portion of the fluid heating zone of the furnace) may range, for example, from 3,000 to 5,000 B. t. u.s per square foot, per hour and the average rate of heating employed for the charging stock in tube bank located within the lower portion of the fluid heating zone may range, for example, from 2,000 to 3,500 B. t. u.s per square foot, per hour.

As an example of one specific operation of the' process of the present invention, a Pennsylvania crude oil of about 45 A. P. I. gravity containing some 37% of gasoline of poor anti-knock value. after being preheated by indirect contact and heat exchange with hot products from within the system, is subjected in a heating coil located within the lower portion of the fluid heating zone of the furnace to a temperature of approximately 550 F., at a superatmospheric pressure of about pounds per square inch and the heated material is introduced into a distilling and fractionating column operated at substantially atmospheric pressure wherein the components of the charging stock boiling up to approximately 500 F., are separated from its higher boiling components. The latter are subjected in the heating coils located adjacent the .floor of the two combustion zones of the furnace to an outlet conversion temperature of approximately 950 F., at a superatmospheric pressure of about 300 pounds per square inch and are then introduced into the reaction chamber which is maintained at substantially the same pressure. The low-boiling fractions of the charging stock. including the gasoline and naphtha fractions boiling up to approximately 500 F., are heated in the tube banks of the furnace adjacent the roof of the two combustion zones to a tempera ture of approximately 1000 F., and are then passed through the tube bank located in the upper portion of the fluid heating zone of the furnace wherein this temperature is substantially maintained, the products being discharged from this zone at a temperature of approximately 980 F., at a superatmospheric pressure of about 800 pounds per square inch into the reaction chamber wherein the pressure is reduced to approximately 300 pounds per square inch. Vaporous and liquid conversion products from the reaction chamber are introduced into alternately operated reduced pressure coking chambers maininch superatmospheric pressure and a regulated portion of the heated products from the light oil heating coil are supplied directly to the lower portion of the coking chambers to assist reduction of the residual conversion products to coke. Vaporous products from the coking chambers tained at approximately 30 pounds per square *0 are subjected to fractionation whereby their insufficiently converted components are condensed as reflux condensate. A selected low-boiling frac tion of the reflux condensate, having a boiling range of approximately 385 to 550 F., is subjected to further conversion, together with the low-boiling fractions of the charging stock. The remaining higher boiling fractions of the reflux condensate are returned to further conversion, together with the high-boiling fractions of the charging stock. This operation'will yield, per barrel of charging stock, about 68% of motor fuel having an anti-knock value equivalent to an octane number of approximately '72 and about 40.pounds of substantially dry coke, the remainder being chargeable principally to uncondensable gas.

I claim as my invention:

1. In a process for the fractional distillation of hydrocarbon oil accompaniedby the pyrolytic conversion of selected fractions thereof under independently controlled temperature and pressure conditions in a heating zone employing a heating coil of the type having two independently fired combustion zones with a fluid conduit located adjacentthe roof and floor of each and with a centrally located fluid heating zone containing fluid conduits heated by combustion gases from both combustion zones, the improvement which comprises passing hydrocarbon oil charging stock for the process through the fluid conduit located in that portion of the fluid heating zone exposed to the mildest heating conditions therein, whereby to heat the oil to a temperature suificient to effect its fractional distillation, passing selected high-boiling fractions of the charging stock through the fluid conduits located adjacent the floor of each combustion zone whereby to rapidly heat the same to the desired conversion temperature, without maintaining the same near the maximum conversion temperature attained for a sufiicient time to permit excessive conversion thereof, and passing selected low-boiling fractions of the charging stock first through the fluid conduits located adjacent the roof of each combustion zone, whereby to heat the same to a high conversion temperature, and then passing the stream of low-boiling highly heated oil through the fluid conduit located in that portion of the fluid heating zone exposed to the most severe heating conditions therein, whereby to maintain the same near the maximum con- Ziersion temperature attained for a predetermined 2. In a process for the fractional distillation of hydrocarbon oil accompanied by the pyrolytic conversion of selected fractions thereof under independently controlled temperature and pressure conditions in a heating zone employing a heating coil of the type having two independent-- 1y fired combustion zones with a fluid conduit located adjacent the roof and floor of each and with a centrally located fluid heating zone also containing fluid conduits heated by combustion gases from both combustion zones, the improvement which comprises passing hydrocarbon oil charging stock for the process through the fluid conduit located in that portion of the fluid heating zone exposed to the mildest heating conditions therein, whereby to heat the oil to a tempcrature suflicient to effect its fractional distillation, passing selected high-boiling fractions of the charging stock through the fluid conduits located adjacent the floor of each combustion zone. whereby to rapidly heat the same to the desired conversion temperature ithout maintaining the same near the maximum conversion temperature attained for a sumcient time to permit excessive conversion thereof, passing selected low-boiling fractions of the charging stock first through the fluid conduits located adjacent the roof of each combustion zone, whereby to heat the same to a high conversion temperature, and then passing the stream of low-boiling highly heated oil through the fluid conduit located in that portion of the fluid heating zone exposed to the most 'severe heating conditions therein, whereby to maintain the same near the maximum conversion temperature attained for a predetermined time, separating the resulting vaporous'and residual conversion products of the process, subjecting the vapors to fractionation whereby their insufficiently converted components are condensed as reflux condensate, subjecting fractionated vapors of the desired end-boiling point to condensation, recovering the resulting distillate, separating the reflux condensate into selected relatively low-boiling and high-boiling fractions, returning the high-boiling fractions to further conversion, together with said high-boiling fractions of the charging stock and returning the lowboiling fractions of the reflux condensate to further conversion, together with said low-boiling fractions of the charging stock.

3. In the heating of hydrocarbon oils in furnaces having a pair of independently fired combustion zones and a convection zone receiving combustion gases from both combustion zones, and each of the combustion zones having floor tubes and roof tubes; the method which comprises heating charging oil for the process to distillation temperature in the convection zone substantially by, convection from the combustion gases and then subjecting the same to fractional distillation, passing selected high-boiling fractions of the charging oil through the floor tubes of each combustion zone and heating the same therein to cracking temperature substantially by radiation under a relatively high rate of heating, and passing selected low-boiling fractions of the charging oil through the roof tubes of each combustion zone and heating the same therein to cracking temperature substantially by radiation under a lower rate of heating than the highboiling fractions in the floor tubes.

4. The method as defined in claim 3 further characterized in that said combustion gases, prior to their use in heating the charging oil, are passed over soaking tubes disposed in the convection zone, said low-boiling fractions being passed through such soaking tubes after the heating thereof in said roof tubes whereby to maintain the same at cracking temperature for a predetermined time.

5. In the heating of hydrocarbon oils in furnaces having a pair of independently fired combustion zones and a convection zone receiving combustion gases from both combustion zones, and each of the combustion zones having floor tubes and roof itubes; the method which comprises heating charging oil for the process to distillation temperature in the convection zone substantially by convection from the combustion gases and then subjecting the same to fractional distillation to form a relatively heavy fraction and a lighter fraction, dividing each of said fractions into two substantially equal streams, passing each stream of the heavy fraction through the floor tubes of one of the combustion zones and beating the'sanie therein to cracking temperature substantially by radiation under a relatively high rate of heating, and passing each stream of the lighter fraction through the roof tubes of one of the combustion zones and heating the same therein to cracking temperature substantially by radiation under a lower rate of heating than the streams of the heavy fraction in the fioor tubes.

6. In the heating of hydrocarbon oils in furnaces having a pair of independently fired combustion zones and a convection zone receiving combustion gases from both combustion zones, and each of the combustion zones having floor tubes and roof tubes; the method which comprises heating charging oil for the process to distillation temperature in the convection zone substantially by convection from the combustion gases and then subjecting the same to fractional distillation to form a relatively heavy fraction and a lighter fraction, dividing each of said fractions into two substantially equal streams, passing each stream of the heavy fraction through the floor tubes of one of the combustion zones and heating the same therein to cracking temperature substantially by radiation under a relatively high rate of heating, and passing each stream of the lighter fraction through the roof tubes of one of the combustion zones and heating the same therein to cracking temperature substantially by radiation under a lower rate of heating than the streams of the heavy fraction in the floor tubes, then combining the streams of the lighter fraction and passing the mixture thereof in indirect heat exchange with the combustion gases in the convection zone prior to the use of the gases in the heating of the charging oil, whereby to maintain the lighter fraction at cracking temperature for a predetermined time.

7. In theheating of hydrocarbon oils in furnaces having a pair of independently fired combustion zones and a convection zone receiving combustion gases from both combustion zones; the method which comprises heating charging oil for the process to distillation temperature in the convection zone substantially by convection from the combustion gases and then subjecting the same to fractional distillation, passing selected high-boiling fractions of the charging oil through a portion of each combustion zone wherein a relatively high rate of heat input prevails and heating the same therein to cracking temperature substantially by radiation under the high rate of heat input, and passing selected lowboiling fractions of the charging oil through another portion of each combustion zone wherein a lower rate of heat input prevails and therein heating the same substantially by radiation to cracking temperature under a lower rate of heat input than the high-boiling fractions.

JOHN WARREN McCAFFREY. 

